设Sn是等差数列{an}的前n项和,S7=3(a2+a12),则a7a4的值为(  ) A.16 B.13 C.35 D.76

问题描述:

设Sn是等差数列{an}的前n项和,S7=3(a2+a12),则

a7
a4
的值为(  )
A.
1
6

B.
1
3

C.
3
5

D.
7
6

由S7=3(a2+a12)结合等差数列的求和公式可得

7(a1+a7)
2
=3(a2+a12),
再由等差数列的性质可得
7×2a4
2
=3×2a7,即7a4=6a7
变形可得
a7
a4
=
7
6

故选D